[Marco van Herwaarden]

To troubleshoot this, please remove any hacks and disable your plugins, then see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

define('DISABLE_HOOKS', true);

If so, reupload all the original vB non-image files (except install.php). Make sure you upload these in ASCII format and overwrite the ones on the server.

If that does not solve your problem, then it is very likely caused by any of the following:
- Not enough memory for PHP
- mod_security installed with a too restrictive config
- Suhosin/Hardened PHP installed with a too restrictive config.